Radio base stations, e.g., of the PHS (Personal Handyphone System) perform communications in a TDMA/TDD (Time Division Multiple Access/Time Division Duplex) method. In the TDMA/TDD method, one frame is formed of four pairs of slots of up and down links. Usually, a control channel is allocated to one of the four slot pairs, and traffic channels are allocated to the three slot pairs. Assuming that “C” represents the control channel, and “T” represents the traffic channel, the frame of the radio base station usually has a frame structure of 1C3T.
Japanese Patent Laying-Open No. 2000-102056 has disclosed a radio base station system formed of a master base station and slave base stations. The frame of the master base station has a usual structure of 1C3T, and the control channel of the master base station is shared by the master and slave base stations. In the slave base station, therefore, traffic channels are allocated to all the four slot pairs. Thus, the frame of the slave base station usually has a structure of 4T. Therefore, the radio base station system has a frame structure of 1C7T as a whole.
In the foregoing radio base station system formed of the master base station and slave base stations, it is not necessary to allocate the control channel to the frame of the slave base station so that it is possible to increase the number the traffic channels, which can be allocated.
When a mobile station makes a call to the radio base station system, the master base station receives a link channel establishment request message from the mobile station, and allocates the traffic channel to the frame of the master or slave base station.
When the traffic channel is allocated to the frame of the master base station, the base station receiving the link channel establishment request is the same as the base station, to which the traffic channel is allocated, so that no problem occurs.
However, when the traffic channel is allocated to the frame of the slave base station, the base station receiving the link channel establishment request is different from the base station, to which the traffic channel is allocated. This causes the following problem.
Even when situations of radio waves of the master base station are suitable for communications with the mobile station, and the master base station can receive the link channel establishment request, such a case may occur that situations of the radio waves of the slave base station are not suitable for the communications with this mobile station, and the exchange of messages and others cannot be performed appropriately between the mobile station and the slave base station. In this case, calling from the mobile station fails.
Accordingly, an object of the invention is to provide a radio base station formed of a master base station and a plurality of slave base stations, and particularly to provide a radio base station system, to which a mobile station can make a call more successfully, as well as a channel allocation method of the radio base station system and a channel allocation program of the radio base station system.